1. Field of the Invention
The present invention relates to devices for measuring temperatures of remote objects, and more particularly to devices for measuring temperatures of crops growing in a field.
2. Prior Art
In recent years, agricultural researchers have had considerable interest in accurately measuring the temperature of crops growing in a field. It has only recently been discovered that the foliage temperature of a plant is directly related to the growth rate. Specifically, it has been discovered that for any given type of plant, there is a narrow foliage temperature range, or a thermal kinetic window, within which the plant will achieve optimum yield and biomass production. Researchers in the field are currently studying the thermal kinetic window of various crops. Such information can be used by a farmer desiring to produce the largest possible harvest. To do this, the farmer attempts to maintain the foliage temperature within this thermal kinetic window for as long as possible to maximize biomass production.
The temperature of plant foliage is dependant upon several factors including the ambient air temperature, wind speed, humidity, solar output and soil water content. As each day in a growing cycle begins, the foliage temperature is typically well below the thermal kinetic window, which is the optimum temperature for biomass production, essentially tracking the ambient air temperature. As the day progresses, the foliage temperature begins to rise as the sun radiates into the field, and the air temperature increases. During the course of a sunny day, the temperature of the ambient air may rise up to and even above the thermal kinetic window. If the foliage temperature were to rise a substantial amount above the thermal kinetic window, the crop may be considered "stressed", in which case the foliage may wilt within a few days, and crop production will suffer due to a decrease in biomass production.
To minimize crop stress caused by an increase in foliage temperature above the thermal kinetic window, additional water is provided to the crop primarily by irrigation. Researchers have found that plants have a capacity (although limited) to maintain their temperature within the thermal kinetic window during periods of increased ambient air temperature. That is, while previously it was thought that plants were "poikilotherms", which means that foliage temperature simply follows the temperature of the environment, it has been discovered that most plants are actually "homeotherms" to a limited extent. A "homeotherm" tries to maintain a temperature within a specified range, similar to a human which for example attempts to maintain the temperature at 98.6.degree. F. Specifically, most plants have a capacity to reduce their temperature by as much as several degrees, in order to maintain a temperature within the thermal kinetic window. To that extent, a plant may be considered a homeotherm. Although plants do have a capacity to reduce their temperature, they have no capacity to raise their temperature, so to that extent they are poikilotherms.
Transpiration, a process comprising the evaporation of water from the surface of the foliage, is an significant method by which a plant can reduce its temperature. In other words, if adequate soil moisture is available the plant can cool itself by several degrees through transpiration in an attempt to maintain its temperature within the optimum temperature range. It has been reported that the temperature within a "crop canopy", which includes air surrounding the leaves cooled by transpiration, may be several degrees below the air temperature, thereby indicating the ability of crops to cool themselves. By reducing the length of time that the crops are subjected to stress caused by elevated temperatures and a lack of soil moisture, an increase in biomass and crop production is possible.
However, since it is often very difficult to determine the amount of water that is sufficient for optimum plant cooling, a farmer may tend to over- or under-water his crop or may fail to apply water at critical times throughout the growing season. Previous methods for calculating the optimum amount of, and time for, irrigation necessary for a given crop required using a complex algorithm that took into account various factors including canopy temperature, air temperature, wind speed, humidity and solar output. Further complicating the watering decision, water may be in short supply or simply unavailable, particularly in periods of drought. Under these conditions, water management is of utmost importance.
Water management is also particularly important to farmers in geographical areas that lack substantial rainfall. In the Western United States, large agricultural concerns rely upon irrigation for all their water needs, and therefore, water is a very precious commodity and efficient management of the water is an absolute necessity for agricultural endeavors.
To maintain optimum crop production by reducing or eliminating crop stress, the farmer must balance the availability and cost of water against his estimate of the needs of the crop. Without detailed knowledge of the foliage temperature of the crop, and without even limited knowledge of the optimum temperature range for his particular crop, the farmers' estimates of the water needs for his crop are often haphazard guesses as to the necessary amount or timing. Thus, there exists a high probability that the crops will not be sufficiently watered for optimum cooling, or will be watered in wasteful quantities.
A farmer having a continuous and accurate measure of crop temperature can immediately respond to the water needs of his crop long before crop damage becomes visibly apparent. If, for example, he observes a decrease in the ability of the crop to effectively cool itself, he will know that more water is needed. If, on the other hand, he observes that the crop is cooling itself effectively, he will know that the crop has sufficient water, and that no more is needed. In times of limited supply, he can use the temperature information to allocate the available water among competing uses and therefore efficiently manage crop growth.
Furthermore, the data obtained regarding foliage temperature may also be used to assist in detecting plant disease. Diseases can cause the plant's stomates or pores through which transpiration occurs, to clog, thereby causing an increase in plant temperature and a corresponding decrease in biomass or crop production.
By correlating knowledge of the individual crop's thermal kinetic window with the foliage temperature, the quantitative degree of plant stress may be readily determined. However, quantification of plant stress in the field is often difficult because of the need to obtain multiple daily measurements of a wide area of plant foliage temperature. Individual foliage temperature measuring devices, such as thermocouples attached directly to the plant are thus an impractical means of obtaining foliage temperature data. Infrared thermometers have been generally employed by the agricultural industry to remotely determine the foliage temperature. Therefore, by measuring the temperature differential between the ambient air temperature and the foliage temperature, a factor relating to the stress of the crop may be determined by the farmer. If stress is indicated, then additional water may be added to the soil through irrigation.
It has been suggested that an infrared thermometer be incorporated in a computer-based sYstem located in the field. This system can measure the crop canopy temperature, air temperature, wind speed, humidity and solar output. Using these measurements, the computer can estimate the crop stress factor.
Typical infrared thermometers utilized to measure foliage temperature have a conical 15.degree. field of view, although instruments having a conical field of view ranging from 4.degree. to 60.degree. are commercially available.
The conical field of view poses a problem for one who wishes to obtain one measurement indicative of the average temperature of the foliage in the field. An infrared thermometer with a narrow field of view can be pointed at one particular place in the field, and a measurement indicative of the temperature at that place can be accurately obtained. However, this measurement may not be indicative of the temperature of the foliage at other places in the field. Thus there is a need for an infrared thermometer with a wide field of view, that receives radiation from a large portion of the field, and provides an output indicative of the average temperature of the foliage in the field.
Infrared thermometers with a conical field of view, such as those currently available, do not satisfy this need. Some may provide a wide field of view, but when used to make temperature measurements, there is an accuracy problem. The difficulty is that the infrared radiation within the wide circular area of the conical field of view of these infrared thermometers often includes background radiation from sources such as the soil and the sky. With this unwanted background radiation incident upon the infrared sensor, the temperature measurements are affected and therefore inaccurate to some degree.